A rotary lawnmower cuts grass by rotating a cutting blade along the grass, the cutting blade being housed in a housing which is open at the bottom. Such lawnmowers include lawnmowers designed to be used in a bagging mode. A lawnmower used in bagging mode stores the grass cut by the cutting blade (i.e., the cut grass) in a bag or another cut grass storage container, and since all of the cut grass can be removed, the results of cutting grass with this lawnmower are superior, and such lawnmowers are widely used in golf courses and the like. Known examples of lawnmowers that can be used in a bagging mode are disclosed in U.S. Pat. No. 4,951,449 and Japanese Patent No. 3771529.
The housing of the lawnmower disclosed in U.S. Pat. No. 4,951,449 is a bagging mode housing in which a grass discharge chute extends rearward and upward from a top wall. By attaching and removing a chute plug to and from the grass discharge chute, the bagging mode housing can be switched between bagging mode and mulching mode. In mulching mode, the cut grass is cut even finer within the housing, discharged out below the housing, and returned to the lawn.
The cutting blade cuts grass by rotating around a shaft substantially perpendicular to the lawn, and creates a revolving flow around the shaft within the housing. When the lawnmower is used in bagging mode, the cut grass is delivered to the grass discharge chute while being made to revolve around the shaft within the housing by the revolving flow. The speed of the revolving flow in the vicinity of the shaft is much lower than the speed of the revolving flow in the outer periphery of the housing. Therefore, the cut grass may stop around the shaft (a problem known as grass buildup can occur). The occurrence of grass buildup must be prevented in order for all of the cut grass in the bag from the housing to be efficiently stored in the bag via the grass discharge chute.
The lawnmower disclosed in Japanese Patent No. 3771529 can be manually switched between a bagging mode for fully opening a shutter and storing cut grass in a cut grass storage container, a mulching mode for fully closing the shutter and discharging the cut grass out below the housing, and an intermediate mode between the housing mode and the mulching mode, wherein the shutter is set to the desired open position. The lawnmower disclosed in Japanese Patent No. 3771529 has the same problems as the lawnmower disclosed in U.S. Pat. No. 4,951,449.